This section provides background information related to the present disclosure which is not necessarily prior art.
It is relatively common in the manufacture of articles, such as automobile bodies, printed circuit boards, and aircraft fuselages, to secure components to fixtures and then to assemble the components together. The fixtures are intended to orient the components relative to one another, in a manner that attempts to position selected features on the components at a nominal position. This practice has significant drawbacks including dimensional variability leading to quality issues, high scrap and a reduced ability to compete in a global market. As an example, variation reduction in the assembly of automotive bodies is a challenging endeavor. A typical automobile body can employ between 100 to 150 sheet metal parts, which can be assembled to one another in a modern, high volume assembly line in 55 to 75 assembly stations. The assembly line can have between 1500 to 2000 fixture locators that are employed to locate various combinations of the components to perform approximately 4000 welds. A single failure that occurs at a locator, a welding spot or from a part that is not fabricated correctly can cause a dimensional variation. This variation will then propagate downstream from station to station and accumulate at the final station of the body assembly. After fabrication/assembly of the body, the vehicle doors, hood, windshield, and trunk lid are then mounted and fit to the body. The dimensional variation is accumulated at the body assembly openings, compounded by variations associated with panels or other subassemblies, and will significantly increase the manufacturing complexity, leading to more tooling failures and unexpected downtime and reduction in both product quality and production throughput. These issues can also cause leaks, noise and risk of water leakage. While there has been much accomplishment in the art of data acquisition using laser sensors and optics, little has been done to utilize the data in real time to impact the dynamic movement of the tooling to reduce variation. Most manufacturing companies continue to rely on human knowledge and analysis to estimate and make tooling corrections (shimming) one locator at a time—after the fact, meaning parts have already been incorrectly joined. It's worthy to note that dimensional problems contribute to approximately two thirds of all quality-related problems during new product launches.
One drawback relates to variation in the manufacturing processes for the body components and the inability of the prior art assembly practice to immediately adjust to the particular tolerance/configuration of a body part loaded into the weld fixture.
Conventional weld fixtures commonly employ locator pins that engage holes in one or more of the body components. As many of the body components are stampings, the diameter of the holes tends to be relatively consistent, but due to variation in the material from which the body components are formed (e.g., chemical composition, micro-structure, thickness of the material) and variation in the processing of the material (e.g., temperature of the stamping die, type/condition of lubrication, amount of lubrication, temperature of workpiece, speed in which the body component is formed), the exact location of these holes tends to move (albeit in a small manner) relative to other features on the body component. It should be appreciated that while the variation is relatively small, the variation of each of the features can compound when the body component is joined to other body components.
Despite the existence of part-to-part variation in a body component, it is common practice in the assembly plants to employ nominal part geometry and/or historic sample data to position a locator relative to a frame of a weld fixture and thereafter employ trial-and-error techniques to reposition (i.e., shim) the locator as needed. Moreover, in an attempt to eliminate small positioning errors between the weld fixture and the body component (which would further contribute to undesired variation in the joining of the body components together), the locator pins are commonly sized to the largest possible or largest statistically probable diameter of the hole in the body component. Consequently, it is not uncommon for the locator pin to engage the hole in the body component in an interference fit manner. Moreover, it is not uncommon, due to the variation in the positioning of the hole(s) in the body component relative to other features, to have the locator pin offset from the hole in the body component (i.e., the axis of the locator pin is offset from the axis or centerline of the hole in the body component). Accordingly, a relatively large amount of stress can be exerted on the body component and the locator pin as a result of the interference between the locator pin and the hole in the body component, and the offset between the axis of the locator pin and the centerline of the hole in the body component. These stresses induce wear on the locator pins, which can essentially machine the locator pins to a smaller diameter, and can cause the locator pin to fail.
When working with conventional locator pin technology, the following scenario is quite common. The manufacturer measures a part after assembly or weldment, invests a large amount of time to review and analyze the collected information only to find that the critical part dimensional features are slightly off. The conventional solution to compensate for the dimensional deviation is to use a shim (flat metal piece of various sizes) pack that is installed to slightly move the location of the offending pin to offset the dimensional error in next part weldment. Often such movements may be on the order of a few millimeters; nevertheless these shims must be used or the part will not be in proper position for welding.
Installing shim packs is labor intensive, not only during the installation process but after installation as well, because meticulous records of where shims were added need to be added to the manufacturing records. Under conventional practice, a shim log is used, requiring a human to make entries in the log about the particulars of each shim added.
In view of the above remarks, there remains a need in the art for an improved process for fabricating a multi-piece assembly that is formed by joining multiple components together.